The present invention relates in general to demodulator circuits and, more particularly, to an FM (frequency modulation) demodulator circuit having an input responsive to an FM signal and first and second outputs in which an alternating current flows. The average value of the alternating current over each half cycle of the FM signal is proportional to the deviation of the FM signal.
Many applications, including home and car stereos and cellular telephones, utilize FM demodulators to recover the information encoded in the FM transmission. Prior FM demodulators have used multiplier circuits and quadrature phase shift networks to produce the demodulated output signal. Typically, the input of the FM demodulator is coupled to the input of the phase shift network which outputs a signal in quadrature with respect to the input signal. The input of the FM demodulator and the output of the phase shift network are coupled to the inputs of the multiplier circuit which provides an output signal proportional to the deviation of the frequency of the FM signal relative to the center frequency of the FM demodulator. Since the amplitude of the output signal of the multiplier is proportional to this deviation in frequency, the FM input signal has been converted to an amplitude modulated (AM) signal which can drive various output devices such as speakers.
A major problem associatd with the most, if not all, prior art arises from the use of the quadrature phase shift network and the multiplier circuit due to the harmonic distortion introduced into the demodulated output signal by these devices. Some FM demodulators employ complex feedback correction circuits to reduce the distortion by increasing the gain and linearity of the multipliers. These correction circuits have only a limited degree of success since it is very difficult to completely remove the harmonic distortion once it has been introduced into a circuit. In addition, the FM demodulator should be responsive to the instantaneous frequency of the FM signal independent of the amplitude variations of the FM signal (AM rejection). Most FM demodulators are sensitive to variations in the amplitude of the input FM signal.
Hence, there exists a need for a new approach to FM demodulation which eliminates the need for phase shit networks and complex feedback circuits. The new approach utilizes an oscillator circuit and a commutator circuit to produce an output signal wherein the average value of the output signal over each half cycle of the FM signal is proportional to the deviation of the operating frequency of the FM input signal from the free-running frequency of the oscillator.